This invention relates to a circuit for generating a PWM wave used for a signal such as a motor driving signal in an R-DAT (rotary head type digital audio tape recorder) and, more particularly, to a circuit of this type capable of shortening a period of a PWM wave without reducing quantization bit number of binary information for producing the PWM wave or shortening a period of a reference clock for the PWM wave.
In an R-DAT, various motors are provided including a drum motor for driving a rotary drum, a capstan motor for driving a capstan and a reel motor for driving a reel support. For these motors, DC motors such as brush motors and brushless motors are used and these motors are controlled in their rotation speed (or rotation speed and rotational phase) through a servo loop.
As a method for controlling the rotation speed of these motors, there is a method using a PWM wave as a drive signal. According to this method, a period of a reference signal is compared with a period of a motor rotation speed detection signal detected from an FG (frequency generator) or a phase of a rotational phase detection signal detected from a PG (phase generator) and duty ratio of a PWM wave is changed in accordance with an error between the two signals (i.e., the duty ratio is increased if the detected speed or phase is slower than a regular speed or phase whereas it is decreased if the detected speed or phase is faster than the regular speed or phase) and the thus adjusted PWM wave is applied to the motor. If the period of the PWM wave is sufficiently short, the motor is driven by an average voltage of the PWM wave so that the speed of the motor increases as the duty ratio increases and it decreases as the duty ratio decreases whereby the speed of the motor is controlled to a predetermined speed.
In a case where binary information is used as information for determining the duty ratio of PWM wave (i.e., error signal in the above described case), a PWM wave is produced in the prior art method by generating, at a certain period, a signal which sustains a state "1" during a period of time corresponding to the value of the binary information (more exactly, period of time which is [value of the binary information].times.[period of reference clock for generating the PWM wave]).
In the prior art method in which a PWM wave which sustains a state "1" during a period of time corresponding to the value of the binary information is produced, a period of PWM wave corresponding to the value of full bits of the binary information is required and, accordingly, the period of the PWM wave becomes longer as the quantization bit number of the binary information increases. If, for example, the quantization bit number of information for determining the duty ratio of the PWM wave is 8 bits, a PWM period of 2.sup.8 .times.[reference clock period] is required.
In a servo loop of the rotation speed or rotational phase of a motor driven by the PWM wave, the period of an error output is longer than the period of the PWM wave due to capacities of the FG and PG so that the PWM wave is generated several times with the same pattern of duty ratio during a period of time from generation of one error output till generation of next error output. When a new error output has been generated, different data (i.e., data corresponding to the preceding error output) is produced for a period corresponding to one period of PWM wave at the maximum before the new error output is provided after being pulse-width modulated. Besides, since the period of the error output and the PWM period are not synchronized with each other, the period of time during which a PWM wave based on different data is produced is not constant but varies each time. If the PWM period is shorter than the error output period to the extend that it can be neglected, there will be substantially no problem. If, however, the PWM period is not sufficiently shorter than the error output period, the error output becomes inaccurate.
For shortening the period of the PWM wave, it is conceivable to reduce the quantization bit number of information for generating the PWM wave or to shorten the reference clock period. The former method, however, is disadvantageous in that the information such as error signal obtained will become less accurate with a result that accurate control of speed or phase cannot be expected. The latter method has a limit in shortening the period of reference clock.
It is, therefore, an object of the invention to provide a PWM circuit capable of shortening the period of a PWM wave without reducing the quantization bit number of binary information for generating the PWM wave or shortening the reference clock period and thereby improving accuracy of an error signal when the circuit is applied to, e.g., a motor drive device.